@charset "utf-8";
* {
				padding: 0;
				margin: 0;
			}
			
			#math {
				margin: 0 auto;
				border: 2px solid #909090;
				width: 500px;
				height: 400px;
			}
			
			#screen {
				height: 50px;
				margin-top: 5px;
				border: 2px solid #f0f0f0;
				text-align: right;
				padding-right: 15px;
			}
			
			#contoller {
				border: 2px solid #f0f0f0;
				margin-top: 20px;
				position: relative;
			}
			
			ul {
				height: 100%;
				width: 100%;
			}
			
			ul::after {
				content: "";
				display: block;
				clear: both;
			}
			
			ul li {
				border: 1px solid skyblue;
				float: left;
				list-style: none;
				width: 24%;
				height: 78px;
				text-align: center;
				line-height: 100px;
				cursor: pointer;
			}
			